malloc: replace instances of __builtin_expect with __glibc_unlikely

malloc: replace instances of __builtin_expect with __glibc_unlikely

Replaced all instances of __builtin_expect to __glibc_unlikely
within malloc.c and malloc-debug.c.  This improves the portability
of glibc by avoiding calls to GNU C built-in functions.  Since all
the expected results from calls to __builtin_expect were 0,
__glibc_likely was never used as a replacement.  Multiple
calls to __builtin_expect within a single if statement have
been replaced with one call to __glibc_unlikely, which wraps
every condition.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>